U.S. patent application number 11/582194 was filed with the patent office on 2008-04-17 for multi-chassis emulated switch.
This patent application is currently assigned to Cisco Technology, Inc., A California Corporation. Invention is credited to Thomas Edsall, Elango Ganesan, Soei-Shin Hang, Lawrence Kreeger, Ramana Mellacheruvu, Sanjay Sane.
Application Number | 20080089247 11/582194 |
Document ID | / |
Family ID | 39302998 |
Filed Date | 2008-04-17 |
United States Patent
Application |
20080089247 |
Kind Code |
A1 |
Sane; Sanjay ; et
al. |
April 17, 2008 |
Multi-chassis emulated switch
Abstract
A solution is provided wherein the interfaces between multiple
chassis (e.g., edge switches) in a network of layer 2 devices and a
spanning tree device are treated as a single emulated switch. This
emulated switch effectively enables two different views to the two
different sides. Thus, frames from the network of layer 2 switches
destined to any port of the emulated switch may take any of the
links (through any of the physical switches), thereby enabling
effective load-balancing for frames traveling from the layer 2
network side into the spanning tree device. Meanwhile the spanning
tree device does not recognize an illegal loop in its connection to
two different edge switches as it views the two links as a single
logical EtherChannel.
Inventors: |
Sane; Sanjay; (Fremont,
CA) ; Kreeger; Lawrence; (Fremont, CA) ;
Edsall; Thomas; (Cupertino, CA) ; Ganesan;
Elango; (Palo Alto, CA) ; Hang; Soei-Shin;
(Saratoga, CA) ; Mellacheruvu; Ramana; (San Jose,
CA) |
Correspondence
Address: |
BEYER WEAVER LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
Cisco Technology, Inc., A
California Corporation
San Jose
CA
|
Family ID: |
39302998 |
Appl. No.: |
11/582194 |
Filed: |
October 16, 2006 |
Current U.S.
Class: |
370/256 ;
370/392 |
Current CPC
Class: |
H04L 12/462 20130101;
H04L 45/48 20130101; H04L 45/00 20130101; H04L 45/245 20130101;
H04L 47/125 20130101; H04L 49/351 20130101; H04L 49/70 20130101;
H04L 45/586 20130101 |
Class at
Publication: |
370/256 ;
370/392 |
International
Class: |
H04L 12/28 20060101
H04L012/28 |
Claims
1. A method for configuring a system of devices including a
spanning tree device connected to two or more edge switches in a
network of layer 2 switches, the method comprising: creating an
emulated switch between the spanning tree device and the two or
more edge switches; and configuring each of the two or more edge
switches such that the emulated switch is viewed as part of the
network of layer 2 switches, equidistant from each of the two or
more edge switches.
2. The method of claim 1, wherein said network of layer 2 switches
is a Data Center Ethernet (DCE) network.
3. The method of claim 1, wherein said spanning tree device is a
Classic Ethernet (CE) device.
4. The method of claim 1, further comprising: periodically
synchronizing data between each of the two or more edge
switches.
5. The method of claim 4, wherein the data includes media access
control (MAC) tables.
6. The method of claim 4, wherein said periodically synchronizing
occurs over an ESL link.
7. The method of claim 1, wherein said configuring includes
configuring each of the two or more edge switches to perform a
self-forwarding check to ensure that a frame received from one port
of the emulated switch is not forwarded to another port of the
emulated switch.
8. The method of claim 1, wherein said configuring includes
configuring each of the two or more edge switches to, upon receipt
of a unicast frame from another switch in the network of layer 2
switches, determine if the frame is for the emulated switch, and if
so, examine a subswitch identification within the unicast frame and
forward the frame to the spanning tree device based upon the
subswitch identification.
9. The method of claim 1, wherein said configuring includes
configuring each of the two or more edge switches to, upon receipt
of a unicast frame from another switch in the network of layer 2
switches, determine if the frame is for the emulated switch, and if
so, examine a local identification within the unicast frame and
forward the frame to the spanning tree device based upon the local
identification.
10. The method of claim 1, wherein said configuring includes
configuring each of the two or more edge switches to, upon receipt
of a broadcast or multicast frame from another switch in the
network of layer 2 switches, coordinate with each of the other of
the two or more edge switches to forward the multicast frame from
only one of the edge switches.
11. The method of claim 10, wherein said one of the edge switches
is selected based on a load balancing algorithm.
12. A method for forwarding a unicast frame from a switch in a
network of layer 2 switches to a spanning tree device via one of a
plurality of layer 2 switches connected to the spanning tree
device, the method comprising: encapsulating the unicast frame with
a header including a switch identification equal to an emulated
switch identification; determining one of the plurality of layer 2
switches connected to the spanning tree device to which to send the
unicast frame based on a shortest path algorithm where an emulated
switch having the emulated switch identification is viewed as
equidistant from each of the plurality of layer 2 switches
connected to the spanning tree device; and forwarding the unicast
frame to the layer 2 switch calculated by said determining.
13. The method of claim 12, wherein the switch is a Data Center
Ethernet (DCE) switch.
14. The method of claim 12, wherein the shortest path algorithm
recognizes links that are inactive or inoperative and determines
the shortest path in light of this information.
15. A method for handling a unicast frame received at a switch in a
network of layer 2 switches from another switch in the network of
layer 2 switches, the unicast frame destined for a spanning tree
device, wherein said spanning tree device is connected to two or
more of the switches in the network of layer 2 switches, the method
comprising: determining that a switch identification in the unicast
frame corresponds to an emulated switch between the switch and the
spanning tree device; stripping a header from the unicast frame,
the header including the switch identification; and forwarding the
unicast frame to the spanning tree device.
16. The method of claim 15, wherein said forwarding includes
forwarding the unicast frame based on a subswitch identification in
the header.
17. The method of claim 15, wherein said forwarding includes
forwarding the unicast frame based on a local identification in the
header.
18. The method of claim 15, further comprising: if the connection
between the switch and the spanning tree device is inoperable,
transferring the unicast frame to one of the other of said two or
more layer 2 switches for forwarding of the unicast frame to the
spanning tree device.
19. The method of claim 18, wherein said transferring occurs via an
ESL link.
20. A method for handling a multicast or broadcast frame received
at a switch in a network of layer 2 switches from another switch in
the network of layer 2 switches, the frame destined for a spanning
tree device, wherein said spanning tree device is connected to two
or more of the switches in the network of layer 2 switches, the
method comprising: coordinating with each of the other of said two
or more switches to determine one switch that will directly forward
the frame to the spanning tree device; if the switch is the one
switch that will directly forward the frame to the spanning tree
device, stripping a header from the frame; and forwarding the frame
to the spanning tree device.
21. The method of claim 19, wherein said coordinating occurs via an
ESL link.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to computer networking.
[0003] 2. Description of the Related Art
[0004] Data management within organizations is an ever increasing
concern, especially with the rise of the Internet information age.
The heart of this data management function is sometimes known as a
data center. Over the last decade, data centers have evolved into
the strategic focus of Information Technology (IT) efforts to
protect, optimize, and grow the organization.
[0005] Data center managers face several challenges in fulfilling
these goals. Most enterprise data centers grew rapidly to meet the
explosive economic growth of recent times. Consequently,
applications commonly stand alone in underutilized, isolated
infrastructure silos. Each infrastructure silo is designed based on
the inclination of the specific application being deployed, so that
a typical data center supports a broad I assortment of operating
systems, computing platforms, and storage systems. The disparate
infrastructures supporting different application "islands" are
difficult to change or expand and expensive to manage, integrate,
secure, and back up. FIG. 1 illustrates this type of "isolated
application" environment.
[0006] One solution to this problem is to design a data center
environment that is highly scalable, resilient, secure, and able to
integrate multiple applications and protocols. One such solution is
known as the Data Center Network Architecture. A specific
implementation of the Data Center Network Architecture is known as
Data Center Ethernet (DCE). DCE allows for consolidation of input
and output, and improved forwarding of communications within the
network. This may be accomplished via specialized protocols and
functionality operated by switches within a DCE network via network
layer 2. Each of the switches within the DCE network may be a layer
2 device. FIG. 2 illustrates a DCE network. Edge switch 200 may be
connected to a server 202. Edge switch 204 may be connected to
server 206. Edge switches 200, 204 may then be connected to several
core switches 208, 210, which then may be connected to other edge
switches 212, 214. Each DCE switch may be assigned a unique
identifier. A routing protocol, such as
Intermediate-System-to-Intermediate-system (IS-IS), may be used
inside DCE. Switches using this routing protocol may append
information to frames sent though the DCE. This appended
information may be in the form of a MAC-in-MAC header attached to
the frame. Edge switches 212, 214 may then each be connected to
non-DCE devices, such as Classic Ethernet (CE) switches 216. CE
switches do not run the forwarding protocols supported by DCE, and
do not append the MAC-in-MAC information. They run a variant of the
Spanning Tree protocol. They are connected to the DCE network.
[0007] Rather than forwarding frames to MAC addresses, DCE switches
send frames to edge switches based on the edge switch
identification via the MAC-in-MAC header. The edge switch then
knows which of its ports to send the frame out to arrive at the
correct MAC address (for example, the port connected to switch
216), and strips off the MAC-in-MAC header prior to doing so.
[0008] The network design depicted in FIG. 2, however, encounters a
problem during actual operation. Specifically, when two links
originate from the same CE switch 216 to different DCE switches
212, 214, the spanning tree protocols operated by CE switches
recognize this as a spanning tree loop. The remedy for such a loop
is to activate only one link at a time. This, however, eliminates
the possibility of load sharing and providing redundancy across 2
(or more) DCE chassis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates a typical "isolated application"
environment.
[0010] FIG. 2 illustrates a typical DCE network.
[0011] FIG. 3 illustrates an example network of layer 2 switches
including an emulated switch.
[0012] FIG. 4 illustrates a standard DCE hierarchical address
format.
[0013] FIG. 5 illustrates an example method for configuring a
system of devices including a spanning tree device connected to two
or more edge switches in a network of layer 2 switches.
[0014] FIG. 6 illustrates an example method for forwarding a
unicast frame from a device in a network of layer 2 switches to a
spanning tree device via one of a plurality of layer 2 switches
connected to the spanning tree device.
[0015] FIG. 7 illustrates an example method for handling a unicast
frame received at a switch in a network of layer 2 switches from
another switch in the network of layer 2 switches.
[0016] FIG. 8 illustrates an example method for handling a
multicast or broadcast frame received at a switch in a network of
layer 2 switches from another switch in the network of layer 2
switches.
[0017] FIG. 9 illustrates an example apparatus for configuring a
system of devices including a spanning tree device connected to two
or more edge switches in a network of layer 2 switches.
[0018] FIG. 10 illustrates an example apparatus for forwarding a
unicast frame from a device in a network of layer 2 switches to a
spanning tree device via one of a plurality of layer 2 switches
connected to the spanning tree device.
[0019] FIG. 11 illustrates an example apparatus for handling a
unicast frame received at a switch in a network of layer 2 switches
from another switch in the network of layer 2 switches.
[0020] FIG. 12 illustrates an example apparatus for handling a
multicast or broadcast frame received at a switch in a network of
layer 2 switches from another switch in the network of layer 2
switches.
DESCRIPTION OF EXAMPLE EMBODIMENTS
Overview
[0021] A solution is provided wherein the interfaces between
multiple chassis (e.g., edge switches) in a network of layer 2
devices and a spanning tree device are treated as a single emulated
switch. This emulated switch effectively enables two different
views to the two different sides. Thus, frames from the network of
layer 2 switches destined to any port of the emulated switch may
take any of the links (through any of the physical switches),
thereby enabling effective load-balancing for frames traveling from
the layer 2 network side into the spanning tree device. Meanwhile
the spanning tree device does not recognize an illegal loop in its
connection to two different edge switches as it views the two links
as a single logical EtherChannel.
Example Embodiments
[0022] In this application, numerous specific details are set forth
in order to provide a thorough understanding of the present
invention. It will be obvious, however, to one skilled in the art,
that the present invention may be practiced without some or all of
these specific details. In other instances, well known process
steps have not been described in detail in order to not obscure the
present invention.
[0023] A solution is provided wherein the interfaces between
multiple chassis (e.g., edge switches) and non-DCE devices are
treated as a single emulated switch. This emulated switch
effectively enables 2 different views to the 2 different sides.
FIG. 3 illustrates an example network of layer 2 switches including
an emulated switch. Here, emulated switch 300 is viewed as being
between edge switches 302, 304 and the CE device 306. It appears to
be equidistant from each of the edge switches 302, 304. For the
CE-side, it appears that the multiple links are actually a single
port bundle (a single EtherChannel). Thus the CE switch/host would
now use the multiple ports in a load-balanced fashion. For the
DCE-side, it makes the multiple links appear as a single port of
the emulated switch, which is physically connected to the multiple
DCE switches that participate with the emulated switch. Thus,
frames destined to any port of the emulated switch may take any of
the links (through any of the physical DCE switches), thereby
enabling effective load-balancing for frames traveling from DCE
side into the CE switch/host.
[0024] DCE switches typically each have their own switch
identifications and independently participate in DCE forwarding. In
an embodiment of the present invention, the edge switches connected
to the non-DCE device co-ordinate the emulated switch
identification that is to be used to represent the multi-chassis
etherchannels. In this embodiment, both physical switches may
advertise their reachability to this emulated switch identification
to the rest of the DCE network. The emulated switch is advertised
as being equidistant from the edge switches. This allows the rest
of the DCE network to build shortest path routes towards the
emulated switch, which pass through one of the edge switches. Since
the shortest path is determined based on physical proximity to the
emulated switch, this effectively load balances between the edge
switches connected to the emulated switch. This is in direct
conflict with the prior art, which simply disabled one of the
links, routing all traffic through only one of the edge switches,
creating a potential bottleneck for the traffic.
[0025] In an embodiment of the present invention, broadcasts and/or
multicasts originating from the DCE network would be allowed only
through one port of this multi-chassis Etherchannel of the emulated
switch. This requires coordination between the physical DCE
switches. Similarly, broadcasts and/or multicasts originating from
a non-DCE network could come into the DCE switches through any of
the etherchannel ports. The decision as to which of the links to
use may be based on some sort of load balancing algorithm. The goal
would likely be to have traffic split evenly among the links. A
Self-forwarding check on the Multi-Chassis EtherChannel ports may
also compare the frame's hierarchical address with the port's
hierarchical address. Therefore, even if the frame ingressing from
one Multi-Chassis EtherChannel port tries to egress out of another
Multi-Chassis EtherChannel ports (of the different physical
switch), it can be dropped due to the self-forwarding check.
[0026] Unicast frames towards hosts belonging to the emulated
switch could come to any of the edge switches connected to the
emulated switch. In an embodiment of the present invention, each of
these physical switches would use its local Multi-Chassis
EtherChannel, i.e. locally attached ports that belong to
Multi-Chassis EtherChannel, to deliver unicasts. If all of the
Multi-Chassis EtherChannel ports on that switch are down, the
frames may be sent to the peer switch over a link such as an
Emulated Switch Link (ESL). The peer switch could then use its
ports belonging to that Multi-Chassis EtherChannel to forward the
frames.
[0027] Addressing using the emulated switch may be accomplished
using a consistent hierarchical address format. This format may be
the standard DCE hierarchical address format. This format is
depicted in FIG. 4. Here, switch ID 400 may be used to store the
emulated switch identification. Then, either the subswitch ID field
402 or the local ID field 404 may be used for Multi-Chassis
EtherChannel. In either case, the physical DCE switches connected
to the emulated switch coordinate so that all ports of the same
Multi-Chassis EtherChannel get the same hierarchical address. It
should be noted that there could be multiple Multi-Chassis
EtherChannels within a single emulated switch-each Multi-Chassis
EtherChannel gets the same emulated switch ID.
[0028] An emulated switch link (ESL) between the edge switches may
be used to enable learning across ports belonging to the
Multi-Chassis EtherChannel, i.e. to synchronize the MAC table for
ports belonging to the Multi-Chassis EtherChannel. The ESL link may
also be used to carry data frames belonging to the Multi-Chassis
EtherChannel ports during link failures, for exchanging emulated
switch control plane messages between the physical switches (LACP
protocol frames, MAC learning updates, co-ordination of switch-id,
LIDs, etc.), or as a regular data link.
[0029] It should be noted that the processes described above need
not be limited to DCE and non-DCE switch combinations. Technically,
the DCE network may be any network of layer 2 switches, while the
non-DCE devices may be any network of devices that uses a spanning
tree or similar algorithm. For purposes of this document, the term
"spanning tree device" will be used to refer to any device that
uses a spanning tree or similar algorithm. In one embodiment, this
device may be a CE device.
[0030] FIG. 5 illustrates an example method for configuring a
system of devices including a spanning tree device connected to two
or more edge switches in a network of layer 2 switches. The
spanning tree device may be a Classic Ethernet switch and the
network of layer 2 switches may be a DCE network. At 500, an
emulated switch may be created between the spanning tree device and
the two or more edge switches. At 502, each of the two or more edge
switches may be configured such that the emulated switch is viewed
as part of the network of layer 2 switches, equidistant from each
of the two or more edge switches. This may include configuring each
of the two or more edge switches to, upon receipt of a unicast
frame from another device in the network of layer 2 switches, check
to ensure the unicast frame is for the emulated switch, and if so,
examine a subswitch or local identification within the unicast
frame and forward the frame to the spanning tree device based upon
the subswitch identification or local identification. The
configuring may also include configuring each of the two or more
edge switches to, upon receipt of a broadcast or multicast frame
from another device in the network of layer 2 switches, coordinate
with each of the other of the two or more edge switches to forward
the multicast frame from only one of the edge switches. The
particular one of the edge switches may be selected based on a load
balancing algorithm. At 504, data may be periodically synchronized
between each of the two or more edge switches. This data may
include, for example, a MAC table. This synchronization may occur
over an ESL link.
[0031] FIG. 6 illustrates an example method for forwarding a
unicast frame from a device in a network of layer 2 switches to a
spanning tree device via one of a plurality of layer 2 switches
connected to the spanning tree device. The device may be a layer 2
switch. At 600, the unicast frame may be encapsulated with a header
including a switch identification equal to an emulated switch
identification. At 602, one of the plurality of layer 2 switches
connected to the spanning tree device to which to send the unicast
frame may be determined. This may be determined based on a shortest
path algorithm where an emulated switch having the emulated switch
identification is viewed as equidistant from each of the plurality
of layer 2 switches connected to the spanning tree device. The
shortest path algorithm may recognize links that are inactive or
inoperative and determine the shortest path in light of this
information. At 604, the unicast frame may be forwarded to the
layer 2 device calculated by the determining.
[0032] FIG. 7 illustrates an example method for handling a unicast
frame received at a switch in a network of layer 2 switches from
another switch in the network of layer 2 switches. The unicast
frame is destined for a spanning tree device, wherein the spanning
tree device is connected to two or more of the switches in the
network of layer 2 switches. At 700, it may be determined that a
switch identification in the unicast frame corresponds to an
emulated switch between the switch and the spanning tree device. At
702, it may be determined if the connection between the switch the
spanning tree device is inoperable. If not, then at 704, a header
may be stripped from the unicast frame, the header including the
switch identification. Then at 706, the unicast frame may be
forwarded to the spanning tree device. The forwarding may include
forwarding the unicast frame based on either the subswitch
identification or the local identification in the header. If the
connection is inoperable, then at 708, the unicast frame may be
transferred to one of the other of the two or more layer 2 switches
for forwarding of the unicast frame to the spanning tree device.
This transferring may occur, for example, via an ESL link.
[0033] FIG. 8 illustrates an example method for handling a
multicast or broadcast frame received at a switch in a network of
layer 2 switches from another switch in the network of layer 2
switches. At 800, the switch may coordinate with each of the other
of the two or more switches to determine one switch that will
directly forward the frame to the spanning tree device. This
coordinating may occur via, for example, an ESL link. At 802, it
may be determined if the switch is the one switch that will
directly forward the frame to the spanning tree device. If so, then
at 804, a header may be stripped from the frame. At 806, the frame
may be forwarded to the spanning tree device.
[0034] FIG. 9 illustrates an example apparatus for configuring a
system of devices including a spanning tree device connected to two
or more edge switches in a network of layer 2 switches. The
spanning tree device may be a Classic Ethernet switch and the
network of layer 2 switches may be a DCE network. An emulated
switch creator 900 may create an emulated switch between the
spanning tree device and the two or more edge switches. An edge
switch configurer 902 coupled to the emulated switch creator 900
may configure each of the two or more edge switches such that the
emulated switch is viewed as part of the network of layer 2
switches, equidistant from each of the two or more edge switches.
This may include configuring each of the two or more edge switches
to, upon receipt of a unicast frame from another device in the
network of layer 2 switches, check to ensure the unicast frame is
for the emulated switch, and if so, examine a subswitch or local
identification within the unicast frame and forward the frame to
the spanning tree device based upon the subswitch identification or
local identification. The configuring may also include configuring
each of the two or more edge switches to, upon receipt of a
broadcast or multicast frame from another device in the network of
layer 2 switches, coordinate with each of the other of the two or
more edge switches to forward the multicast frame from only one of
the edge switches. The particular one of the edge switches may be
selected based on a load balancing algorithm. An edge switch
synchronizer 904 coupled to the edge switch configurer 902 may
periodically synchronize data between each of the two or more edge
switches. This synchronization may occur over an ESL link.
[0035] FIG. 10 illustrates an example apparatus for forwarding a
unicast frame from a device in a network of layer 2 switches to a
spanning tree device via one of a plurality of layer 2 switches
connected to the spanning tree device. The device may be a layer 2
switch. A unicast frame header encapsulater 1000 may encapsulate
the unicast frame with a header including a switch identification
equal to an emulated switch identification. A layer 2 switch
unicast frame route determiner 1002 coupled to the unicast frame
header encapsulater 1000 may determine one of the of the plurality
of layer 2 switches connected to the spanning tree device to which
to send the unicast frame. This may be determined based on a
shortest path algorithm where an emulated switch having the
emulated switch identification is viewed as equidistant from each
of the plurality of layer 2 switches connected to the spanning tree
device. The shortest path algorithm may recognize links that are
inactive or inoperative and determine the shortest path in light of
this information. A unicast frame forwarder 1004 coupled to the
layer 2 switch unicast frame route determiner 1002 may forward the
unicast frame to the layer 2 device calculated by the
determining.
[0036] FIG. 11 illustrates an example apparatus for handling a
unicast frame received at a switch in a network of layer 2 switches
from another switch in the network of layer 2 switches. The unicast
frame may be destined for spanning tree device, wherein the
spanning tree device is connected to two or more of the switches in
the network of layer 2 switches. An emulated switch identification
determiner 1100 may determine that a switch identification in the
unicast frame corresponds to an emulated switch between the switch
and the spanning tree device. A unicast frame header stripper 1102
coupled to the emulated switch identification determiner 1100 may
strip a header from the unicast frame, the header including the
switch identification. An inoperable spanning tree device
connection determiner 1104 coupled to the unicast frame header
stripper 1102 may determine if the connection between the switch
the spanning tree device is inoperable. If not, then at a unicast
frame spanning tree device forwarder 1106 coupled to the inoperable
spanning tree device connection determiner 1104 may forward the
unicast frame to the spanning tree device. The forwarding may
include forwarding the unicast frame based on either the subswitch
identification or the local identification in the header. If the
connection is inoperable, then a unicast frame layer 2 switch
transferrer 1108 coupled to the inoperable spanning tree device
connection determiner 104 may transfer the unicast frame may be
transferred to one of the other of the two or more layer 2 switches
for forwarding of the unicast frame to the spanning tree device.
This transferring may occur, for example, via an ESL link.
[0037] FIG. 12 illustrates an example apparatus for handling a
multicast or broadcast frame received at a switch in a network of
layer 2 switches from another switch in the network of layer 2
switches. The frame may be destined for spanning tree device,
wherein the spanning tree device is connected to two or more of the
switch in the network of layer 2 switches. A frame spanning tree
device layer 2 switch coordinator 1200 may coordinate with each of
the other of the two or more switches to determine one switch that
will directly forward the frame to the spanning tree device. This
coordinating may occur via, for example, an ESL link. A spanning
tree frame forwarding switch determiner 1202 coupled to the frame
spanning tree device layer 2 switch coordinator 1200 may determine
if the switch is the one switch that will directly forward the
frame to the spanning tree device. If so, then a frame header
stripper 1204 coupled to the spanning tree forwarding switch
determiner 1202 may strip a header from the frame. A spanning tree
device frame forwarder 1206 coupled to the frame header stripper
1204 may forward the frame to the spanning tree device.
[0038] Although illustrative embodiments and applications of this
invention are shown and described herein, many variations and
modifications are possible which remain within the concept, scope,
and spirit of the invention, and these variations would become
clear to those of ordinary skill in the art after perusal of this
application. Accordingly, the embodiments described are to be
considered as illustrative and not restrictive, and the invention
is not to be limited to the details given herein, but may be
modified within the scope and equivalents of the appended
claims.
* * * * *